The present invention relates generally to a radiation emitting device, and more particularly to a linear accelerator having a monolithic cavity structure with asymmetric coupling.
Linear accelerators are used to accelerate a variety of particles (e.g., electrons, protons, ions) for numerous applications, such as radiation therapy. A radiation therapy device generally includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. An electron linear accelerator is located within the gantry for generating a high energy radiation beam for therapy. This high energy radiation beam may be an electron beam or photon (x-ray) beam, for example. During treatment, the radiation beam is trained on a zone of a patient lying in the isocenter of the gantry rotation.
Linear accelerators may be used in the medical environment for a variety of applications. A beam of charged particles, e.g., electrons, from a linear accelerator may be directed at a target which is made of a material having a high atomic number, so that an X-ray beam is produced for radiation therapy. Alternatively, the beam of charged particles may be applied directly to a patient during a radiosurgical procedure. Such radio surgery has become a well-established therapy in the treatment of brain tumors. A high-energy beam may be directed at a localized region to cause a breakdown of one or both strands of the DNA molecule inside cancer cells, with the goal of at least retarding further growth and preferably providing curative cancer treatment.
A conventional linear accelerator includes a series of accelerating cavities that are aligned along a beam axis. A particle source, which for an electron accelerator is typically an electron gun, directs charged particles into the first accelerating cavity. As the charged particles travel through the succession of accelerating cavities, the particles are focused and accelerated by means of an electromagnetic field. For example, a radio frequency (RF) source may be coupled to the accelerator to generate the necessary field to operate the linear accelerator. The accelerated particles from a clinical linear accelerator have a high energy (e.g., up to 20 MeV). Often, the output beam is directed to a magnetic bending system that functions as an energy filter. The beam is typically bent by approximately 270 degrees. Then either the output beam of high energy particles or an X-ray beam generated by impinging a target with the output beam is employed for radiation treatment of a patient.
The frequency of the driving signal and the dimensions of the accelerating cavities and the beam passages between adjacent accelerating cavities determine the operating frequency of the accelerator. Optimal performance of the accelerator requires a match between the resonant frequency of the cavity structure and the frequency of the driving signal.
In a resonant chain of coupled cavities such as used in a standing-wave linear particle accelerator, it is often desirable to change the field strength in some cavities relative to other cavities. Adjustment of the field strength profile in an accelerator can be done by changing the coupling constants on each side of a coupling cavity. This is typically done by shifting the side cavity""s longitudinal position, which makes the coupling aperture larger on one side and smaller on the other. In doing this, the side cavity""s shape is generally unchanged. The side cavity remains symmetrical. This conventional method works well for accelerator designs where the side cavity is manufactured as one piece and attached to a piece which contains two main cavity halves.
An alternative method for manufacturing the accelerator structures is to form monolithic members such as disclosed in U.S. Pat. No. 5,734,168, by Yao, which is incorporated herein by reference in its entirety. The monolithic structure defines a portion of the main cavity and side cavity in one structure. The monolithic structure provides improvements in manufacturing such as reduced tolerances and reduced manufacturing costs, especially for higher frequency accelerators. One drawback with the monolithic structure is that the field strength adjustment as described above cannot be used. If the side cavity is shifted longitudinally, the unit cell will not contain exactly one half of a side cavity, and the frequency of this partial side cavity will be significantly shifted from the frequency of the full side cavity. This complicates the design and testing of cavities.
There is, therefore, a need for a monolithic cell structure that allows for adjustment of the field strength by modifying the side cavity configuration to vary the coupling constant between a side cavity and a main cavity.
A device for use in a linear accelerator operable to accelerate charged particles along a beam axis is disclosed. The device includes a plurality of monolithic members connected to form a series of accelerating cavities aligned along the beam axis and coupling cavities. Each of the coupling cavities intersects with adjacent accelerating cavities at first and second coupling apertures. The first and second coupling apertures have different sizes.
In another aspect of the invention, a system for delivering charged particles for medical applications generally comprises a particle accelerator having an input for connection to a source of charged particles and a plurality of accelerating cells. The particle accelerator has a beam path extending through the cells to an exit window. Each of the particle accelerating cells comprises an accelerating cavity half cell and a coupling cavity half cell. The particle accelerating cells are connected to form a series of accelerating cavities aligned along the beam axis and coupling cavities. Each of the coupling cavities intersects with adjacent accelerating cavities at first and second coupling apertures. The first and second coupling apertures have different sizes. The system further includes a signal source for energy transfer engagement with the charged particles within the particle accelerator.
The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.